EP0652669B1 - Combined modulator schemes for spatial light modulators - Google Patents

Description

• This invention relates to a method of gray scale printingusing spatial light modulator arrays, and a printeror print engine for performing this method.
• Printing systems trying to achieve gray scale can use spatial lightmodulators in several ways. The arrays usually have to be defined to aspecific dimension, making each type of printing application require adifferent device. Desktop electrophotographic printing using highly sensitivemedia, at 600 dots-per-inch (dpi), needs one configuration of a device ordevices, whereas a computer to plate offset system using relativelyinsensitive media needs another. This translates into high cost, low volumeproduction runs. Yet, the needs of different systems must be met.
• The computer to plate systems need large arrays that can time delayand integrate (TDI) . The image data for one line on the board passes fromline to line on the device, keeping the image data for that line on the boardthe entire time the board is under the device. This gives high energy transferonto the photosensitive media, which does not have very high sensitivity. Atypical array for this application may have as many as 256 rows. Thisappears satisfactory for printing, allowing 256 gray shades.
• However, desktop printing has a different problem. Because of thecurvature and movement of the drum, it is nearly impossible to optically"wrap" a 256-row device around it. Yet, 256 rows of gray scale gives printingsystem users what they need. Therefore, some way must be developed toallow a smaller device to achieve gray scale, and still make those samedevices compatible with systems that have different needs.
• EP-A-0 321 143 discloses a color printing system employing alarge area pattern of controllable light sources, where thepattern includes rows that are aligned substantially withthe movement of a print medium in relation to the lightsources. All of the light sources in each row contribute tothe exposure of each pixel in a corresponding row of theresulting color print according to the TDI-principle.
• US-A-4 074 319 discloses a method for developing a pictorialfield display from transmitted facsimile data. The display,which comprises an array of two level energy sources such aslight emitting diodes, achieves multitone operation bydigitally controlling the number of times during which theenergy sources are activated according to the TDI-principle.
• EP-A-0 467 048 discloses a method ofaddressingan array of bistable deformable mirror elementsusing pulse width modulation for producinga pictorial presentation on a TV-screen in an HDTV-system.
SUMMARY OF THE INVENTION
• The present invention disclosed herein comprises a method and a printer to combinepulse width modulation with time delay and integration (TDI) techniquesthat increase the number of gray scale available for gray scale printing. Theinvention allows using a smaller device to produce many shades of gray,allowing the devices to be used in tandem for other printing applications. Themethod and the printe or print engine in accordancewith this invention are claimed inclaims 1 and 8.
BRIEF DESCRIPTION OF THE DRAWINGS
• For a more complete understanding of the present invention and forfurther advantages thereof, reference is now made to the following DetailedDescription taken in conjunction with the accompanying Drawings in which:
• Figures 1a-1c show a prior art method of Time Delay and Integrationin computer to plate offset printing.
• Figures 2a-2c show a spatial light modulator array performing pulsewidth modulation and TDI at the beginning of a page.
• Figure 3 shows a spatial light modulator array performing pulse widthmodulation and TDI in the middle of a page.
• Figure 4 shows a wafer with spatial light modulator arrays before theyare separated into individual chips.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• Figures 1a-1c show a prior art example of Time Delay and Integration(TDI) in computer to plate offset printing. The light rays impinge uponspatial light modulator 102. One cell shown inrow 104 is activated, in thiscase shown as being deflected. Some examples of such modulators areDigital Micromirror Devices (DMD), Liquid Crystal Devices (LCD), andmagneto-optic modulators. For discussion purposes only, the modulatorshown is assumed to be a DMD, comprising an array of individual mirrorsthat are deflectable such as is shown in Figure 1a.
• The light ray impinging upon deflectedelement 104 passes throughthe lens and strikes theplate 110 atspot 112. The plate is moving in thedirection shown byarrow 108. In Figure 1b, an element inrow 106 isdeflected and the light passes through the lens and strikes thesame spot 112on theplate 110. The movement of the data fromrow 104 to 106 betweenFigures 1a and 1b is coordinated with the movement of theplate 110 in theprocess, so the same data onrow 104 in Figure 1a is now onrow 106 inFigure 1b.
• Figure 1c shows another step in the process. The data that had beeninrow 106 in Figure 1b is now atrow 114 in Figure 1c. The deflectedelement passes the light through the lens and again strikes thespot 112 onplate 110. This allows multiple time exposures of the same spot on the plateas it passes by the modulator, or as the modulator passes by the plate.
• Figures 2a-2c show a method that achieves a larger number of grayshades than simple time delay and integration (TDI). The printing processmoves in the direction ofarrow 208. The face of spatial light modulator isbeing viewed from what is seen as the vertical edge next tomodulator row104 in Figures 1a-1c.
• The modulator selected must have a relatively fast response time. For600 dots-per-inch (dpi), the modulator has to load and display its dataquickly. For example, assume a printer that prints 40 pages per minute,allowing 440 inches per minute, or 7.3 inches per sec. At 600 dpi, one inchhas 600 pixels, requiring the modulator to print 4400 pixels per sec. Fordiscussion purposes, the device configuration is assumed to be 2600 columnsand 128 rows (600 dpi x 8.5" equals 5100 columns of pixels, so there are twodevices of 2600 columns each). Worst case would be if the entire device 2600x 128, or 332,800 pixels would have to be loaded at once. Printing 4400 pixelsper sec x 332,800, results in a 1.46 GHz data rate, a very difficult rate toachieve.
• However, if the modulator were divided up into blocks, where eachblock loads its own data, the data rate can be reduced dramatically. Forinstance, if the data rate upper bandwidth was set at 25 MHz, which is aneasily managed data rate, the modulator would be divided into 58 blocks.The number of frames per page equals 600 dots/inch x 11.5 inch/page, or6900 frames/page, and 6900 frames/page x pages/sec equals the frames persecond. A highly competitive page rate would be 40 pages per minute, or40/60 pages/sec. This results in a frame rate of 4600 frames per second. Theresulting frame time equals 217 µsecs per frame.
• A least significant bit (LSB) time of 217 µsecs means that with nopulse-width modulation, the device would have to be loaded in 217 µsecs. Toperform PWM, where the bits are displayed for a time slice proportional totheir significance, the LSB time would be divided by the number of bits ofPWM. For example, if 2 bits of PWM is desired, the LSB time becomes 217µsecs/2, or 108 µsecs. For faster modulators, this time presents no significantproblems with loading the device. A 2-bit system by 128 TDI would provide256 shades of gray.
• In Figure 2a, the modulator is shown at the very beginning of thecycle. The first row encountered in theprocess direction 208 displays thedata for the most significant bit (MSB) for Row 0 of the printed image. InFigure 2b,row 204 on the modulator displays the LSB for the same row.Data line 0 then moves to the next row on the modulator in Figure 2c. TheMSB for Row 0 is now onmodulator row 206, and the MSB forRow 1 is onmodulator row 204. This process continues up the device. Note that in thiscase there is no optical reversion as in Figures 1a-1c.
• Figure 3 shows an example later in time on a portion ofmodulator 302.The process still moves in direction of thearrow 308.Modulator row 306 isnot modulating the data for the MSB ofRow 63, and row 304 of themodulator displays the MSB ofRow 64. The rest of the modulator's 128 rowswould have the bottom 64 rows displaying Rows' 1-64 data onto the drum.
• It is a distinct advantage of this method that it allows higher levels ofgray than previously achievable. In no way are these levels restricted tomonochrome applications. It can be utilized in color systems. Examples ofsystems that can use this method are offset printing plate, photographic filmprinters, photographic paper printers, and systems using a xerographicengine, such as fax machines, desktop printers, and copiers.
• Another advantage besides the increase of available gray scale is theability to utilize more devices from the manufacturing process. Figure 4shows an illustration of this advantage at the wafer level. Thewafer 402 hasa set of modulators that are essentially finished except for the final dicing ofthe wafer. The modulators would be tested while on the wafer. If forexample, modulators 404a and 406a both tested correctly, they could be leftconnected together and used in the PCB system, which requires high energytransfer. The two would essentially become a 2600 x 256 modulator array, inan embodiment where the modulators were 2600 x 128.
• However, if themodulator 406a tested poorly, andmodulator 404btested correctly, then modulators 404a and 404b would remain linked and gointo a 600 dpi printing system. They would function as a 5200 x 128 array.They can still compete in the gray scale area because of the above technique,requiring fewer lines of a modulator for 256 shades of gray. In the abovesituation, where 406a is a inoperative modulator, and 404a and 404b staylinked for printing,modulator 406b appears to go to waste. However,because of the above techniques, it can still achieve 256 shades of gray, butat 300 dpi. The single modulators can be used in lower-end printing systems.

Claims (8)

1. A method of gray scale printing utilizing a light source;a spatial light modulator comprised of an array ofindividually controlled elements arranged in an x-y grid ofrows and columns so as to receive light from said source;and a moving photosensitive surface arranged so as toreceive modulated light from said individually controlledelements in their on state; said method furthercomprising the steps of:

   pulse width modulating data within each row of saidmodulator, wherein said modulator elements receive andrespond to said data by assuming their on or off states, and saiddata are comprised of at least two bits of differingsignificance received serially by said elements, eachelement responding to said bits by assuming the statedictated by the current bit for a period of timeproportional to the significance of that bit;

   row integrating said pulse width modulated data byaddressing the modulator such that successive rows of saidmodulator receive the same pulse width modulated data for a same line onsaid photosensitive surface, causing the data for that lineon the photosensitive surface to move across the face of themodulator synchronized with the movement of saidphotosensitive surface; and

   imaging light from the on states of the elements of themodulator to said photosensitive surface through alens.
2. The method of claim 1, wherein said pulse widthmodulation is performed on a digital micromirror device.
3. The method of claim 1, wherein said pulse widthmodulation is performed on a liquid crystal device.
4. The method of any preceding claim, wherein saidphotosensitive surface is a drum in a xerographic printer.
5. The method of any of claims 1 to 3, wherein saidphotosensitive surface is an offset printing plate.
6. The method of any of claims 1 to 3, wherein saidphotosensitive surface is a photographic film.
7. The method of any of claims 1 to 3, wherein saidphotosensitive surface is a photographic paper.
8. A printer or print engine arranged to perform the methodclaimed in any preceding claim.

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